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Proceedings of the Combustion Institute 000 (2016) 1–8 www.elsevier.com/locate/proci
Self-ignition and smoldering characteristics of coal dust accumulations in O2/N2 and O2/CO2 atmospheres Dejian Wu a, Martin Schmidt b,∗, Xinyan Huang c, Filip Verplaetsen d a Department b Division
of Mechanical Engineering, KU Leuven, Celestijnenlaan 300A, B3001 Leuven, Belgium 2.2 ‘‘Reactive Substances and Systems’’, BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, Berlin, Germany c Department of Mechanical Engineering, University of California, Berkeley, CA 94720, US d Adinex NV, Brouwerijstraat 5/3, B 2200 Noorderwijk, Belgium Received 3 December 2015; accepted 5 August 2016 Available online xxx
Abstract The self-ignition of coal dust deposits and its subsequent smoldering combustion pose a high fire hazard to oxy-fuel power systems which burn fuels using pure oxygen for the sake of carbon capture and storage. The increasing risk of explosion in the gas-phase and self-ignition in the solid-phase for an oxygen enhanced combustion environment has not been well studied yet. In this work, the heterogeneous reactions of a bituminous coal dust are investigated by using a novel hot-basket apparatus with an emphasis on the roles of O2 and diluent gas in chemisorption and smoldering. Experiments show that increasing O2 mole fraction accelerates both self-ignition and the following smoldering combustion. On the other hand, the presence of CO2 increases the ignition temperature and reduces the maximum smoldering temperature. However, the promotion in the fire and explosion risk by elevating O2 mole fraction is substantially stronger than the retardation effected by presence of CO2 . The emission-gas measurements show that the CO to CO2 ratio increases significantly after self-ignition, and CH4 counts for 1–8% of the total carbon emission. This research may help improve the understanding of heterogeneous coal combustion and the fire safety in oxy-fuel power systems. © 2016 by The Combustion Institute. Published by Elsevier Inc. Keywords: Oxy-fuel combustion; Fire hazard; FTIR; CO/CO2 ratio; CH4
1. Introduction Oxy-fuel combustion, as one of the most promising technologies for carbon capture and ∗
Corresponding author. Fax: +49 3081041227. E-mail address:
[email protected] (M. Schmidt).
storage, has generated remarkable interest for newly built and retrofitted coal-fired power plants [1,2]. Instead of using air as the oxidizer, oxyfuel combustion uses pure oxygen (O2 ) or a mixture of O2 and recycled flue gas (mainly the carbon dioxide (CO2 )) to generate high-concentration CO2 gas product for carbon storage [1–3]. However, the increasing risk of explosion in the gas-phase [4] and self-ignition in the solid-phase [5] for an
http://dx.doi.org/10.1016/j.proci.2016.08.024 1540-7489 © 2016 by The Combustion Institute. Published by Elsevier Inc.
Please cite this article as: D. Wu et al., Self-ignition and smoldering characteristics of coal dust accumulations in O2 /N2 and O2 /CO2 atmospheres, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.08.024
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D. Wu et al. / Proceedings of the Combustion Institute 000 (2016) 1–8
O2 -enhanced combustion environment is still a technological challenge and has not been well studied yet. Dust accumulation often occurs in coal mills and power plants. Once in contact with the oxidant and a mild heat source, the accumulated dust may self-heat to initiate smoldering fires [5–8] which can further generate hot spots to trigger dust explosion. Such smoldering fire risk may vary with increased O2 mole fraction (XO2 [%]) and the presence of CO2 -rich diluent gas. However, little is known about the combined effect of XO2 and CO2 diluent gas on the self-ignition characteristics and the subsequent smoldering combustion behaviors. The typical self-ignition initiated by the selfheating and the following smoldering are governed by the O2 supply and heat losses in the porous fuel bed [6,9–11]. Therefore, the diffusive transportation of O2 is a fundamental problem in heterogeneous combustion dynamics and is of great interest in industrial fire and explosion safety [4]. The O2 mass flux was found to change the smoldering burning rate approximately linearly, but has a weak effect on the forced ignition process [9]. The selfignition temperature (SIT) for XO2 lower than 30% in nitrogen was found to increases with reduced XO2 for both hardwood sawdust [11] and coal dust [12]. The effect of XO2 on smoldering combustion of bench-scale solid fuels has been investigated in both self-heating ignition [13] and forced ignition by an external heat flux [14–16]. Malow and Krause [13] observed that the mass loss rate decreased as XO2 decreased from 21% to 3.6% for wood chips, lignite coal and recycling materials. Walther et al. [14] found that the ignition delay of polyurethane foam decreased with increasing XO2 . Hadden et al. [15] demonstrated the effect of XO2 on the competition between pyrolysis and oxidation of peat, and found that as XO2 decreased, the mass-loss rate decreased while the residual mass and smoldering duration increased, which recently verified by numerical simulations [16]. The carbon emission characteristics are studied for self-ignition of different fuels [13,17] and smoldering under different intensities [18]. On the other hand, the effect of CO2 on coal particle combustion has also been investigated in oxy-fuel atmospheres. Qiao et al. [19] measured the ignition temperatures of coal dust in a wire-mesh reactor in both O2 /N2 and O2 /CO2 atmospheres, and found that the influence of CO2 was smaller than that of XO2 particularly for bituminous coals. In the mg-scale thermogravimetric analysis (TGA) using the same XO2 , the maximum and average mass-loss rates of coal sample in CO2 diluent gas were found to be lower than those in N2 [20,21]. In addition, higher CO production was observed for coal combustion in O2 /CO2 due to the char gasification by CO2 [20]. Despite of using different experimental scales and setups, the ignition and devolatilization behaviors of coal dust in the mixture
Table 1 Properties of SA coal dust sample [4,5]. Properties Proximate analysis Fixed carbon (%) (by diff.) Volatile matter (%) Moisture content (%) Ash (%) Ultimate analysis (on a dry basis) Carbon (%) Hydrogen (%) Nitrogen (%) Sulfur (%) Ash (%) Oxygen (%) (by diff.) Gross heating value of dry fuel (MJ/kg) Bulk density of dry fuel (kg/m3 )
SA coal 56.3 26.9 2.7 14.1 67.5 4.26 1.76 0.44 14.9 11.58 27.37 600 ± 5
of 30% O2 /70% CO2 were observed to be similar to those in air [5,20–22]. In the previous work [5], the self-ignition temperatures of three coal dusts were determined using both hot-oven and hot-plate tests in air and O2 /CO2 mixture. In this work, a novel hot-basket apparatus coupled with a scale and Fourier transform infrared (FTIR) spectrometry was employed. By varying XO2 in diluent gas (N2 or CO2 ), we first measure the temperature, mass loss and gas emission of bench-scale coal dust samples, and then analyze the combined effect of O2 and diluent gas on the self-ignition and subsequent smoldering combustion. 2. Experimental 2.1. Coal dust samples and atmosphere compositions A dried medium volatile bituminous coal from South Africa (SA), same as previous work [4,5], was investigated. The coal sample was first milled or sieved into dust with the average diameter less than 63 μm, and then dried in a vacuum oven at 80 °C until the maximum moisture content was lower than 3% on the mass basis. The properties of SA coal are summarized in Table 1. In the experiment, XO2 is set to 21, 30, 40 and 50 ± 1% in both O2 /N2 and O2 /CO2 ambient. 2.2. Experimental setup The basic experimental apparatus included of a laboratory oven following the standardized basket test [13,23]. An inner chamber of about 40 L in volume was installed inside of the regular heat storage oven to better control the flow near coal dust samples, as illustrated in Fig. 1. Two equidistant cylindrical mesh wire baskets with volumes of 100 mL and 400 mL were used. For each basket,
Please cite this article as: D. Wu et al., Self-ignition and smoldering characteristics of coal dust accumulations in O2 /N2 and O2 /CO2 atmospheres, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.08.024
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Fig. 2. Proposed reaction pathways proceeding in the self-ignition and smoldering of coal [25,28]. Fig. 1. Schematic diagram of the hot-oven test coupled with TG-scale and FTIR.
both temperatures at the center (Tcb ) and the half of the distance between the center and outer surface (Thcb ) were measured by two K-type thermocouples. An additional thermocouple was located outside of the basket to monitor the oven ambient temperature (Ta ) in the inner chamber. A scale (Makro-TG, ± 0.01 mg) was installed to measure the sample mass evolution. The sample in the basket was hung by a hook with an oscillating tungsten wire, and its mass was instantaneously measured through the frequency of oscillations. The fixed gas mixture flow of 5 L/min was monitored by a flowmeter, and was preheated with a copper pipe coil before entering the inner chamber. The oven temperatures were selected to be slightly higher than the previously found SITs in [5] to ensure a smoldering ignition. The flue gas out of the inner chamber was directed to an FTIR spectrometer for analysis. During the experiment, XO2 of the flue gas was measured continuously by an O2 sensor. The data collection system was used to simultaneously record the temperature evolution, sample mass, and FTIR spectrometer for mole fractions (Xi ) of O2 , CO2 , CO, CH4 , and H2 O(g) in the flue gas. Note that XCO2 in the O2 /CO2 gas mixtures exceeded the upper limit (40%) of FTIR, thus the increment of CO2 in O2 /CO2 mixtures was roughly estimated as XCO2 = 100% − XCO2 ,0 − XO2 − XCO − XCH4 − XH2 O(g) where XCO2 ,0 is the carbon dioxide mole fraction in the inlet stream.
3. Kinetics of self-ignition The possible reaction pathways for lowtemperature oxidation and high-temperature smoldering are proposed in Fig. 2. The interaction between coal and O2 at low temperature (
